Confirmed -- But I am having the opposite issue. I can check the "editable" checkbox for the project polygon layer, but not for my editable feature layers! I even tried to publish just a single editable point feature service and the "editable" checkbox will not activate. I can edit this feature service in the Web Map with no issues, but when I try to do so via the Cost Analysis tool is does not work. Interestingly enough, if I add an editable feature service that I published several months ago, I can activate the "editable" checkbox, but not for one I published this morning. Too bad because this tool holds a lot of promise.